1. Field of the Invention
The present invention relates to an apparatus and method for measuring a surface shape of an object having a test surface (i.e. a surface under test) using an interferometer that images interference fringe images generated from test lights (i.e. lights reflected surface under test) and reference lights (i.e. lights reflected reference surface flat). More particularly, the present invention relates to an attitude control system for an apparatus that employs a plurality of interference fringe images caused from lights phase-shifted between the test surface and reference surface (i.e. reference flat surface).
2. Description of the Related Art
In an interferometer, an interferogram can be obtained between a test light and a reference light(standard surface) that is incorporated in the interferometer. An optical surface shape measurement apparatus known in the art employs the interferometer to take interference fringe images and analyzes the images to measure a plane surface shape.
In order to control the attitude of the test surface relative to the reference surface in the interferometer, an obliquity between the test surface and the reference surface is determined in the art based on a fringe interval or fringe direction in the interferograms. In this case, at least two interferograms are required to determine the obliquity. In addition, whereas the degree of the obliquity can be found from a single interference fringe image, it is not possible to even find the direction of the obliquity. It is required for finding the direction of the obliquity to confirm the direction of a deviation of the interferogram when the attitude of the test surface is slightly varied. Thus, an operator in the art is required to manually adjust the attitude along with viewing the interferograms.
On the other hand, if two or more interferograms can be seen as described above, it indicates that the test surface relatively tilts from the reference surface and this situation is not suitable for a high accuracy measurement. In an interferogram analysis measurement, desirably an angle between the test surface and the reference surface is near zero as far as possible. Nevertheless, if the test surface is a flat plane and its angle to the reference surface is near zero, the interval between the interferograms extends over the whole observing surface, causing a state a null-interference that exhibits an entirely uniform, bright or dark screen. It is not possible in this state to execute a high accuracy attitude control for adjusting an extremely small oblique angle between the test surface and the reference surface.
As described above, the conventional apparatus for interferogram analysis measurement has a disadvantage because the attitude control of the test surface relative to the reference surface is not easy. In addition, when a high accuracy surface shape measurement is desirable to be performed in the state of null-interference, which has a small obliquity between the almost flat test surface and the reference surface, the attitude can not be controlled disadvantageously.
The present invention accordingly has an object to provide an optical surface shape measurement apparatus and method capable of controlling the attitude of a test surface relative to a reference surface in an automatic manner with a high accuracy using an interferogram measurement by the phase shift method.
The present invention is provided with an optical surface shape measurement apparatus using an interferometer. The apparatus comprises a phase shift interferometer for imaging at least three interference fringe images each generated by emitting a coherent light to an object having a test surface and to a reference having a reference surface so that a test light reflected from the test surface interferes with a reference light reflected from the reference surface while varying a phase difference between the test light and the reference light from image to image; a surface shape analyzer for analyzing the at least three interference fringe images taken by the phase shift interferometer to obtain surface shape information of the test surface; a typical plane computer for computing a typical plane of the test surface on the basis of the surface shape information obtained from the surface shape analyzer; an oblique angle computer for computing an oblique angle of the typical plane computed by the typical plane computer to the reference surface; and an attitude adjusting mechanism for automatically adjusting the attitude of the test surface relative to the reference surface on the basis of the oblique angle computed by the oblique angle computer.
The present invention is also provided with an optical surface shape measurement method using an interferometer. The method comprises the steps of: imaging at least three interference fringe images each generated by emitting a coherent light to an object having a test surface and to a reference having a reference surface so that a test light reflected from the test surface interferes with a reference light reflected from the reference surface while varying a phase difference between the test light and the reference light from image to image; analyzing the at least three interference fringe images to obtain surface shape information of the test surface; computing at typical plane of the test surface on the basis of the surface shape information; computing an oblique angle of the typical plane to the reference surface; and automatically adjusting the attitude of the test surface relative to the reference surface on the basis of the oblique angle.
According to the present invention, the use of the phase shift interferometer allows an obliquity of the test surface to be obtained even if an angle between the test surface and the reference surface is near zero. This makes it possible to realize an optical surface shape measurement apparatus with the interferogram measurement, which an control the attitude of the test surface relative to the reference surface in an automatic manner with a high accuracy.
According to the present invention, the use of the phase shift interferometer allows an obliquity of the test surface to be obtained even if an angle between the test surface and the reference surface is near zero. This makes it possible to realize an optical surface shape measurement apparatus with the interferogram measurement, which can control the attitude of the test surface relative to the reference surface in an automatic manner with a high accuracy.
The phase shift interferometer in the present invention may, for example, (a) image at least three interference fringe images with time differences by slightly displacing the reference surface along the optical axis; and (b) image at least three interference fringe images with time differences by variably controlling the wavelength of the coherent light. These cases only require a single imaging device.
Alternatively, the phase shift interferometer may comprise an original beam generator for generating an original beam by superimposing the test light and the reference light within the same observing surface in a state of null-interference; a beam splitter for splitting the original beam generated from the original beam generator into three output beams; an interference system for allowing the test light to interfere with the reference light while varying a phase difference between said test light and said reference light for each of the three output beams obtained from the beam splitter; and three imaging devices for imaging interferograms regarding the three output beams obtained from the interference system.